[Elecraft] OT a bit: K3 Ant Tuner

[Bob McGraw K4TAX]

A bandpass filter is designed for optimum rejection and pass frequencies 
only when operated at 50 ohms.  Thus both ends of the filter need to see 
50 ohms.  If the antenna has a SWR issue, no matter how small, it is not 
likely 50 ohms.  And if one uses the transceiver internal ATU to the 
input of the filter, likewise the filter input doesn't see 50 ohms.

Correctly used the bandpass filter must be in the path between the PA 
output and the ATU input.  With an internal ATU, this isn't likely 
possible.

And most filters are designed for a certain power level when operated 
from a source and into a load of 50 ohms.  Should one depart from the 
correct source and load impedance, then the filter my fail due to 
excessive voltage.

If the K3 is seeing an SWR of 2:1 or less, don't bother.  If the 
operator feels good about "touching it up" that is only good for the 
operator.  The radio doesn't care one way of the other.

Use a bandpass filter correctly or don't use one.  For if you do, you'll 
likely let the smoke out of the box.

73

Bob, K4TAX


On 7/5/2019 9:57 AM, Rich wrote:
> To minimize emails direct replies would be nice.  I searched the web 
> and could not find an answer.   I know there are a ton of smart folks 
> on this list so I thought I would ask.
> TU
> On Field Day at typical setup is :
>
> K3 (or any radio) - bandpass filter -  External Antenna Tuner  - Antenna
>
> So the antenna was tuned via the external tuner, but saw a guy then 
> using the K3 ant tuner to touch up the SWR between the radio and the 
> bandpass filter.  Is that an acceptable practice?
>
> I would think not?
>
> Rich
>
> K3RWN
>
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